IsSubnameAvailable function is used to check for the availability of a subname before minting is attempted. Subnames which are already registered cannot be minted.

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IsL1SubnameAvailable

Used to check the availability of a subname when minting happens on the L1 Chain. 
public async isL1SubnameAvailable(
  subname: string,
): Promise<boolean>

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IsL2SubnameAvailable

Used to check the availability of a subname when minting happens on the L2 Chain. 
public async isL2SubnameAvailable(
  subname: string,
  chainId: number
): Promise<boolean>